This is a good overview of Git internals. If this stuff interests you, Chapter 10 of Pro Git offers similar descriptions of Git objects [1] and Git references [2], and then continues onto Git packfiles [3] which are not covered by OP.

[1]: https://git-scm.com/book/en/v2/Git-Internals-Git-Objects

[2]: https://git-scm.com/book/en/v2/Git-Internals-Git-References

[3]: https://git-scm.com/book/en/v2/Git-Internals-Packfiles